Attack of the Teleclones

[Sean McGrath]

PHYSICS NEWS UPDATE                                                     	
The American Institute of Physics Bulletin of Physics News
Number 765  February14, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                 	
ATTACK OF THE TELECLONES: Should quantum cryptographers begin to
worry?  In contrast with everyday matter, quantum systems such as
photons cannot  be copied, at least not perfectly, according to the
"no-cloning theorem."  Nonetheless, imperfect cloning is permitted,
so long as Heisenberg's Uncertainty Principle remains inviolate.
According to Heisenberg, measuring the position of a particle
disturbs it, and limits the accuracy to which its complementary
property (momentum) can be determined, making it impossible to
reliably replicate the particle's complete set of properties.
Now, quantum cloning has been combined with quantum teleportation in
the first full experimental demonstration of "telecloning" by
scientists at the University of Tokyo, the Japan Science and
Technology Agency, and the University of York (contact Sam
Braunstein, schmuel at cs.york.ac.uk and Akira Furusawa,
akiraf at ap.t.u-tokyo.ac.jp). In ideal teleportation, the original is
destroyed and its exact properties are transmitted to a second,
remote particle (Heisenberg does not apply because no definitive
measurements are made on the original particle).  In telecloning,
the original is destroyed, and its properties are sent to not one
but two remote particles, with the original's properties
reconstructed to a maximum accuracy (fidelity) of less than 100%.
(Heisenberg limits the ability to make clones as otherwise
researchers could keep making copies of the original particle and
learn everything about its state.)
In their experiment, the researchers didn't just teleclone a single
particle, but rather an entire beam of laser light. They transmitted
the beam's electric field, specifically its amplitude and phase (but
not its polarization) to two nearly identical beams at a remote
location with 58% accuracy or fidelity (out of a theoretical limit
of 66%).  This remarkable feature of telecloning stems from the very
magic of  quantum mechanics: quantum entanglement. Telecloning
stands apart from local cloning and from teleportation in requiring
"multipartite" entanglement, a form of entanglement in which
stricter correlations are required between the quantum particles or
systems, in this case three beams of light.  (An example of a
multipartite entanglement is the GHZ state between three particles
that was featured in Update 414.)
In addition to representing a new quantum-information tool,
telecloning may have an exotic application: tapping quantum
cryptographic channels. Quantum cryptographic protocols are so
secure that they may discover tapping.  Nonetheless, with
telecloning, the identity and location of the eavesdropper could be
guaranteed uncompromised. (Koike et al., Physical Review Letters, 17
February 2006; for an earlier partial demonstration of telecloning,
between an original photon and one clone at a remote location and
another clone local to it, see Zhao et al., Phys Rev Lett, 13 July
2005)

[...]

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